Split handwritten tests by resource instead of CLI / read / unit

[melinath]

The existing CLI / read / unit split is confusing for users, and each test file has too much magic in it, which can make it hard to understand what's actually happening. We should instead split tests by resource and prioritize readability. It might help to do this by generating the tests automatically, which would require marking all resources exclude_validator: true - similar to #232.

Blocked by #332

[melinath]

sort of related: b/196008728

[marktru]

Internal issue: b/196008728